1. Physiology. The normal rhythmically occurring relaxation and dilatation of the heart chambers, especially the ventricles, during which they fill with blood.
2. The lengthening of a normally short syllable in Greek and Latin verse.

[Greek diastolē, dilation, separation, from diastellein, to expand : dia-, apart; see dia- + stellein, to place, send.]

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.